Paraxylene is a commodity chemical that is oxidized to terephthalic acid and purified to produce purified terephthalic acid, an intermediate in the manufacture of polyester. In 2001, the total world installed capacity of paraxylene was approximately 21 MMTA (million metric tons per annum), and 4.3 MMTA in the U.S.
The production of paraxylene begins with a suitable substantially hydrocarbon feedstock. As used herein, such a substantially hydrocarbon feedstock suitable for the production of paraxylene can come from several sources, and can be categorized into high and low paraxylene concentrations. The term “substantially hydrocarbon feedstock” refers to a hydrocarbon feedstock that comprises and often consists essentially of ethylbenzene, paraxylene, metaxylene, orthoxylene, and optionally aromatic and aliphatic hydrocarbon impurities.
Most feedstocks from which paraxylene is recovered are derived from catalytic reforming processes found in many petroleum refineries. The reformate product generally comprises C6 to C11 aromatics wherein the C8 aromatics fraction generally comprise metaxylene, orthoxylene, paraxylene and ethylbenzene. Other byproducts of the reforming reaction are hydrogen, light gas, paraffins, naphthenes and heavy C12+ aromatics. Paraxylene-containing feedstocks may also include pyrolysis gasoline, conventional toluene disproportionation products, conventional transalkylation products, and the intra-stage products of paraxylene recovery processes.
In all of these feedstocks, the xylene isomers are generally near their equilibrium distribution, which is about 25% paraxylene, about 50% metaxylene, and about 25% orthoxylene. The C8 distillation fraction of these feedstocks generally comprises all of the C8 aromatic isomers due to the close proximity of their boiling points. The low equilibrium concentration of paraxylene is further diluted by the presence of ethylbenzene, such that the C8 fraction derived by distillation from reformate (reformate xylenes) typically comprises from about 10 to about 20 weight percent ethylbenzene, and more typically from about 15 to about 18 weight percent ethylbenzene. The C8 fraction of pyrolysis gasoline typically comprises as much as about 30 to about 60 weight percent ethylbenzene, whereas the C8 fraction of conventional toluene disproportionation typically comprises only about 2 to about 7 wt % ethylbenzene. Dilution by ethylbenzene and the equilibrium distribution of the xylene isomers reduces the paraxylene content of these feedstocks to as low as about 10 to about 25 weight percent paraxylene, with reformate xylenes typically comprising about 15 to about 20 weight percent paraxylene. It is understood that these feedstocks may be preprocessed to selectively remove metaxylene or orthoxylene, which would increase the paraxylene concentration. Thus, substantially hydrocarbon feedstocks with low paraxylene concentrations generally comprise less than about 50 weight percent paraxylene, commonly less than about 30 weight percent paraxylene, and from time to time less than about 20 weight percent paraxylene.
Substantially hydrocarbon feedstocks with high paraxylene concentrations generally comprise at least about 50 weight percent paraxylene, commonly at least about 70 weight percent paraxylene, and from time to time at least about 80 weight percent paraxylene. Substantially hydrocarbon feedstocks with high paraxylene concentrations arise from sources including feedstocks produced by Selective Toluene Disproportionation (STDP), selective alkylation, or selective transalkylation, as described in U.S. Pat. No. 4,097,543 and U.S. Pat. No. 4,117,026, and in W. W. Kaeding, et al., J. Catal., 67, 159 (1981). They are also found in the second or subsequent stages of multi-stage crystallization processes for recovering paraxylene from substantially hydrocarbon feedstocks with low paraxylene concentration. Substantially hydrocarbon feedstocks with high paraxylene concentration also include the paraxylene-enriched stream produced in the selective adsorption zone of a hybrid adsorption/crystallization paraxylene process, such as that described in U.S. Pat. No. 5,329,060.
Two processes useful for recovering paraxylene are low temperature crystallization, and selective adsorption on a molecular sieve. “Parex” is the most widely applied molecular sieve adsorption process, as described in D. P. Thornton, Hydrocarbon Proc. 49 (1970) at pp. 151-155, which is incorporated herein by reference. This process is based on the principle of continuous selective adsorption in the liquid phase employing fixed beds of solid adsorbent. The adsorbent is made from a zeolite, and the separation technique is based on small differences in affinity to the adsorbent. Paraxylene has the strongest affinity to the adsorbent and is thus preferentially adsorbed. The affinity of the desorbent liquid is positioned between those of paraxylene and the other feed components. When the desorbent affinity is too low, it will take a lot of effort to remove the paraxylene from the adsorbent. If the affinity is too high, the paraxylene is not capable of displacing the desorbent from the adsorbent. Furthermore, the volatility of the desorbent should differ sufficiently from that of the feed compounds to allow for separation of the paraxylene-desorbent and non-paraxylene-desorbent mixtures by distillation.
A crystallization process can also be used to recover paraxylene from a substantially hydrocarbon feedstock. Such a paraxylene crystallization process comprises an isomerization section, a fractionation section, and a crystallization section. Alternatively, such a crystallization process comprises a section for producing a substantially hydrocarbon feedstock with high paraxylene concentration, a fractionation section, and a crystallization section.
To efficiently recover a purified paraxylene from such substantially hydrocarbon feedstocks, the crystallization process includes one or more crystallization stages which generally comprise, jacketed crystallizers, which are typically scraped wall vessels with refrigerated jackets through which a vaporizing refrigerant passes. The crystallization stages may also comprise at least one reslurry drum. The crystallization stage may also comprise a scraped wall heat exchanger, where the material being crystallized is passed through a scraped tube side of the heat exchanger and vaporizing refrigerant is passed through the shell side of the heat exchanger.
Ethylene is often used as a refrigerant to recover paraxylene in paraxylene crystallization processes that use a substantially hydrocarbon feedstock, since efficient paraxylene recovery from these dilute paraxylene streams requires temperatures as low as about −90° F. Ethylene vapor generated from ethylene liquid in the crystallization stages from heat transfer from the material being crystallized exhibits a vapor pressure that is still above atmospheric pressure at these low temperatures, and thus can be conveniently used in a typical vapor recompression refrigeration loop. When using a hydrocarbon refrigerant such as ethylene, it is desirable that the refrigerant vapor pressure is above atmospheric at the temperatures encountered in the crystallization section to prevent oxygen ingress from potential leaks, which could lead to explosive mixtures.
However, since the critical temperature of ethylene is about 49° F., it is generally not possible or practical to condense ethylene in heat exchangers via air or water cooling. Thus, the ethylene vapor exiting the crystallizers is compressed and condensed by exchange with another refrigerant, typically propane or propylene. The propane or propylene circulates in another vapor recompression loop, and the propane or propylene vapor is condensed by air or water cooled exchangers. Thus, a typical practice of a paraxylene crystallization process uses this cascaded ethylene/propane or ethylene/propylene refrigeration circuit.
U.S. Pat. No. 3,177,265 issued to Lammers discloses a multi-stage crystallization process for recovering paraxylene from a C8 or mixed xylene feed, wherein ethylene refrigerant is used to cool the first stage, and propane is used to cool a subsequent stage. Commercial variations of the Lammers process also utilize propane refrigerant for condensing and optionally desuperheating the ethylene refrigerant for other miscellaneous refrigeration requirements such as trim cooling of the feed to the crystallizers, and cooling of the fractionation system off-gas to improve recovery of benzene into the light aromatics byproduct stream.
U.S. Pat. No. 5,448,005 issued to Eccli et al. discloses a crystallization process for paraxylene recovery where a single temperature crystallization production stage is used for producing paraxylene from a feed having an above equilibrium paraxylene concentration, such as from a selective toluene disproportionation process. The process uses a refrigeration system to provide cooling to a temperature of about −20° F., and propane or propylene can be used for the refrigerant.
The above processes utilize propane or propylene vapor compression refrigeration loops for the recovery of paraxylene. In these loops, the propane or propylene vapors produced by the transfer of heat are subsequently compressed to a higher pressure after which the vapors can be condensed by air or water cooled heat exchangers. The propane or propylene refrigeration compressors are costly, complex and inefficient machines requiring a substantial amount of energy to operate. Reducing the size of or eliminating these machines would substantially reduce the capital cost required to build paraxylene units as well as reduce energy consumption and operating costs. These processes also require costly heat exchange equipment to perform refrigerant cooling in addition to the high cost of the coolant utilities themselves. Thus, there is a need for a more cost effective mechanism for performing refrigeration in paraxylene recovery processes.
Ammonia Absorption Refrigeration (AAR) can be a cost-effective, energy saving process that has been used for providing moderate temperature refrigeration. In AAR, an enthalpy source such as waste heat reboils an ammonia fractionator that is fed a stream enriched in ammonia relative to water, (also shown as strong aqua in FIG. 6). The fractionator separates the stream enriched in ammonia relative to water into a higher purity ammonia vapor overhead stream, and a stream enriched in water relative to ammonia (also shown as weak aqua in FIG. 6) bottoms stream. The ammonia vapor overhead stream is condensed via air or water cooling to liquid ammonia refrigerant. The liquid ammonia refrigerant is then directed to the refrigerant users. As enthalpy is transferred indirectly from the material being refrigerated, the liquid ammonia refrigerant evaporates and generates ammonia refrigerant vapor. The ammonia vapor is directed to an absorber, along with the stream enriched in water relative to ammonia which absorbs the ammonia vapor while releasing heat of absorption. The heat of absorption is typically removed by water cooling the absorber.
U.S. Pat. No. 4,116,652 issued to Zondek discloses a process for freeze concentration of liquid mixtures including solutions by directly contacting the mixtures with a miscible refrigerant in at least two stages. Zondek discloses that good results are obtained when the product being concentrated is xylene and the refrigerant is ammonia. The process also permits the selected separation of crystals at two or more temperatures where crystals can be removed from a first stage and also from a second stage or succeeding stages.
U.S. Pat. No. 4,331,826 issued to Kagawa discloses a process comprising (1) mixing a paraxylene containing feed with an inert liquid refrigerant, (2) feeding the mixed liquid into a lower part of a bubble tower type crystallization tower, (3) evaporating the refrigerant to form a slurry of p-xylene crystals in the paraxylene feed, (4) separating the inert refrigerant as a vapor from the liquid surface of an upper part of the crystallization tower, and (5) separating paraxylene crystals from the slurry discharged from the upper part of the crystallization tower. Examples of inert refrigerant include carbon dioxide, ammonia, and lower hydrocarbons having 2 to 4 carbon atoms.
Both the '652 and '826 patents disclose direct contact crystallization processes in which the refrigerant is injected directly into the solution being crystallized. Direct contact cooling with a refrigerant requires costly capital equipment and expenditures in energy to efficiently separate the refrigerant from the hydrocarbon streams being cooled. For this reason, direct contact cooling has found limited commercial applications.
In the context of other pure product or byproduct directed processes, the product must be stripped substantially free of all residual ammonia. In the context of paraxylene recovery, the ammonia refrigerant would need to be reduced to on the order of 1 ppmw from the crystallization section reject filtrate, since it is a severe poison to the acid catalysts used in paraxylene unit isomerization sections.
As a result of these and other challenges, AAR has not been successfully applied for the separation of paraxylene from mixed aromatics.
It has now been found that indirect AAR, in accordance with the present invention, can be utilized to provide cooling for paraxylene crystallization while overcoming the problems realized from processes where ammonia was added directly to the feedstocks.
It has also been found that refrigeration derived from an AAR process comprising certain vaporization, water/ammonia contacting, separating and revaporizing steps in accordance with the present invention, results in substantially improved process economics for paraxylene crystallization than traditional processes relying on capital and energy intensive propane vapor recompression refrigeration systems.
It has also been found that several enthalpy sources distinct to a paraxylene crystallization process can power the integrated AAR in accordance with the present invention, resulting in a substantial savings in energy costs and reduction of undesirable emissions of carbon dioxide to the atmosphere.